LayoutTests/http/tests/misc/form-blob-challenge.html

<script>
if (window.testRunner) {
    testRunner.dumpAsText();
    testRunner.waitUntilDone();
}

var formData = new FormData();
formData.append("data", new Blob(["blobdata"]));

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {
   if (xhr.readyState === 4) {
        alert("Response: " + xhr.responseText);
        if (window.testRunner)
            testRunner.notifyDone();
    }
};
xhr.open("POST", "resources/basic-echo-post.py", true, "testuser", "testpassword");
xhr.send(formData);
</script>